1. Field of the Invention
The present invention relates to a light source apparatus with a MOPA constitution that comprises a seed light source and an optical amplifier.
2. Related Background Art
As industrial laser light sources such as machine laser light sources in the fields of electronics and mechanics, medical laser scalpel laser light sources, length-measuring or other measurement laser light sources, pulse oscillation laser light sources that employ Q switches have been widely used. A Q switch obtains pulse light with a high peak power by modulating the resonance loss of the resonator. As the method of implementing the Q switch, a method that effectively modulates the reflectance of a reflective mirror that constitutes a resonator by means of an optical switch due to the acousto-optic (AO) effect, for example, is the most general and widely prevalent.
A optical fiber laser light source that employs a Q switch has a constitution in which pumping light in the 975 nm wavelength band is supplied to a Yb-doped optical fiber (YbDF) that is an optical amplification waveguide doped with yttrium (Yb), for example. ASE (Amplified Spontaneous Emission) light is emitted by YbDF for which an inverted population is formed through the supply of this pumping light. Normally, RF power is not applied to an AO switch and the ASE output of the YbDF is outputted to the non-reflecting end as zero-order diffraction light. However, when RF power is applied to the AO switch, the direction of the ASE output of the YbDF is changed by the AO diffraction effect. Thereupon, the ASE light is irradiated onto a reflective mirror that is disposed at the tip of the YbDF. Hence, laser resonance is emitted. Further, the energy that has accumulated as a result of the formation of an inverted population in the optical amplifier waveguide is released at one burst such that a high-power optical pulse is produced.
Further, the Q switch laser light source is not limited to the optical fiber laser light source above. A fixed laser light source and gas laser light source can also be implemented. The optical fiber laser light source has a small space coupling part, and is highly reliable and compact. Further, the optical fiber laser light source is practical because the beam quality is stable and good. However, as compared with a fixed laser light source (particularly a disk laser light source), the optical fiber laser light source is generally long with a resonator length of a few meters. For this reason, with the Q switch, the optical propagation time of the resonator is several tens of nanoseconds and there is also the drawback that an optical pulse of a shorter width cannot be produced.
Normally, the energy of the laser light irradiated onto the machine object in laser machining changes from the electrons to lattice vibration and then to heat. However, the shift of energy from the lattice into heat is mostly produced on a time scale of about 10 nanoseconds. Hence, when pulse laser light with a pulse width that is shorter than the time scale is irradiated, adiabatic machining of the machine object is possible. For example, in the case of drilling, an effect such as a small burr at the outer perimeter of the hole is obtained. In order to shorten the pulse width of the Q switch, an example in which the resonator length is kept at 70 cm also exists in J. Limpert, et al, “Sub-10 nsQ-switched Yb-doped photonic crystal fiber laser”, CLEO2005, Tech.Dig., JWB51 (Document 1). However, when the alignment of the lens system required in front of and behind the AO switch and the difficulty when welding the YbDF (a margin on the order of 40 to 50 cm is required at one end for placement in the welder) are considered, it is said to be unrealistic.
As a technology for counteracting the drawbacks of the optical fiber laser light source, a light source apparatus with a MOPA (Master Oscillator Power Amplifier) constitution is known. The light source apparatus with a MOPA constitution has a constitution that amplifies the light that is outputted from the seed light source by means of an optical amplifier and is able to output high power light. In other words, A. Babushkin, et al., “Multi-kilowatt peak power pulsed fiber laser with precise computer controlled pulse duration for materials processing”, Proc. of SPIE Vol. 5709, pp 98 to 102 (Document 2) describes a constitution that amplifies pulse light of a wavelength of 1064 nm that is outputted from the semiconductor laser light source by means of an optical amplifier such that a high-power pulse light output with a pulse repetition frequency of 10 to 140 kHz, a pulse width of 4 to 250 nanoseconds, an average power of 8.5 W, and a peak power of 40 kW is obtained. However, A. J. W. Brown, “Fiber laser development at Aculight Corporation”, 63rd Laser Machining Scientific Society compendium, 2005 (Document 3) describes a constitution in which pulse light outputted from a seed light source is amplified by an optical amplifier such that a pulse light output with a high pulse repetition frequency of several hundred kHz to several hundred MHz is obtained.